Why is one of my carbs keep overflowing with gas?

I'm working on a '79 650B3... it's a bike that sat in a garage since 2002 and I got it in November 2013. I did a full carb break down, clean and re-assembly. First time I fired up the engine was about 2 weeks ago and it runs "good". I plan on carb syncing and further tuning the idle and have started doing some of that work before getting hold of a carb sync gauge.

Well, the bike was running with a rich fuel smell, enough to be uncomfortable standing next to it for more than a couple minutes. I didn't notice fuel dripping from the #1 carb overflow hose until recently (mostly because the bike is in a tight space in my garage with a sometimes wet concrete floor). I know my carbs are close to sync because I adjusted each to equal measurement, however without having the engine running that's neither here nor there.

I set the mix screw on #1 carb to 2 turns out (it was close to it already) and gas was still coming out in bursts just about every minute. It's not the mix screw.

Well, my 79's petcock has RESERVE at the top position, PRIME to the right and ON to the bottom. I had it on RESERVE since there isn't much gas in the tank.

Switching the petcock to ON, stopped the leak from the overflow hose. It's the first time I'm running this bike on ON and I'm glad I found something that stopped the #1 carb from leaking. I also went through the petcock a couple months ago and made sure it was clean and the O ring was good. It all checked out.

I understand the information previously posted and the float was first on my list. Also considered the #1 carb not getting enough air and flooding, lack of spark on the #1 plug, Exhaust pressure and backfire since my exhaust connector pipe is off. Also have no air filter in the air box.

I'd like to get a better understanding of why this is happening, is it the carb or the petcock or a compression problem?

Many thanks

BTW: make sure you get a glass jar under there and collect the gas spilling out. I was surprised how much was coming out. My guess around 1/4 cup every 5 minutes on idle.

Did the clear tube test and it sit right at the line where the bowl meet the carb as described in the manual. The little red plug for the pilot jet is very loose. I'm gonna replace the o ring that is perished. Gonna go to true value tomorrow. Sure they have one. It's so loose it can just fall out. Btw it took about 30-40 min before it started leaking. Any one have any ideas?

Usually the fuel level should be a few mms below the carb body. Maybe another member with your model can confirm this

Actually at a second glance you are right. The position I speak of is the max it should be. Let adjust it and see what happens.

Found this post on the " clear tube test " for reference.

After reading the progress with your problem, I also went back in the garage to take another look at my bike and will be conducting the " clear tube test " just to be certain.

Also a good idea to check the oil for traces of gasoline

It's seems suspect the petcock on my bike was the cause. As mentioned the bike just fired recently and has not run but on the RESERVE setting. I switched it to ON and the leak on the #1 carb stopped. Went back out and rotated the petcock dial a few times for it to settle in, and now it's no longer leaking in the RESERVE setting either.

Replaced the oring on the plug with a new one. Fits snug now. Adjusted the float and it read the same. The leak started fasterand more gas came out. Any ideas? Idle mixture screw? The needle seat?

You may have to replace the needles and seats. The metal can get scratched up and won't seal off gas anymore

The clear tube is the only real way to see what is going on with your needles and seats. I personally would replace the needles and seats with fresh ones just to be certain(be aware this may not be needed). When performing the clear tube test, I like to have the rack of carbs attached to a makeshift stand of some sort. A couple old intake boots screwed to a board that is level and plumb work great. Once you are happy with the float level, tie the tube in place and let it sit for ten minutes or so, just to be sure nothing bleeds through. I chased my tail for a long time tuning my carbs, and only found good results when I started with new parts, and went from there. You may have problems with gasoline deteriorating the hardware store o-rings(I have). Z1enterprises should be able to set you up with any parts you need. Surely there are other sources, they are just the first and best that come to mind.

Be sure there in not a plugged or clogged vent tube or vent hole on the carb before you go to to much trouble. your float has to breath to work clogs wont allow float to meter fuel correctly.

Where is this vent hole at? Btw when I got the bike about a month ago I dissembled the carbs and used a can of carb cleaner to go through everything I could. They should be pretty clean at this point. Still maybe I missed this hole you speak of.

This is a situation that happened to me... very similar to yours:

I recently had the fuel tank off while I was inspecting my KZ650-B2 and changing the spark plugs. In doing so I inadvertently switched the plug wires going to cylinders 2 & 3 from one coil to the other. Once I had everything back together, the bike was very hard to start and it seemed like the #2 cylinder was not firing. To add to the problem it was now leaking fuel from the #3 carb drain tube.

I messed with this situation for a week trying to figure out why this was happening. I even went out and bought new coils. In doing so I read that the number 2&3 cylinders have to be on the coil that has the green wire coming from the points. It was then that I realized my mistake with the plug wires and correctly placed them on the proper plugs.

After this the bike started with half a kick and no more gas leaking from the carb drain tube. I hope this helps in trying to correct your problem.